Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
UMTS is a third generation mobile communication network, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
In LTE the downlink is based on orthogonal frequency division multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM).
The E-UTRAN is made up of eNB nodes, which are connected to each other via the X2 interface. Both the S1 and the X2 interface may be divided into control plane and user plane parts.
Handover, or equivalently handoff, of user equipments between different cells is a key feature in mobile communication networks. In order to avoid unnecessary handovers of user equipments from a first cell to a second cell, a HandOver Margin, (HOM), may be used.
The HOM is the difference between the radio quality of the serving cell and the radio quality needed before attempting a handover i.e. when a handover is triggered. The radio quality may be measured either using Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ).
The user equipment may trigger an intra-frequency handover procedure, i.e. a handover within the same frequency and usually between geographically separated base stations, by sending an event report to a serving base station. This event occurs when the user equipment measures that the target cell is better than the serving cell with a HOM. The user equipment is configured over Radio Resource Control (RRC) when entering a cell, and the HOM is calculated from the following configurable parameters:HOM=Ofs+Ocs+Off−Ofn−Ocn+Hys Where:Ofs is a frequency specific offset of the serving cellOcs is a cell specific offset of the serving cellOff is an offset in radio quality between the serving cell and a neighbor cell.Ofn is a frequency specific offset of the neighbor cellOcn is a cell specific offset of the neighbor cellHys is the hysteresis for entering and leaving the event A3 condition
The different events, such as event A3 above, for handover is further described in 3GPP TS 36.331 V10.5.0 (2012-03), section 5.5.4—Measurement report triggering.
Thus it is possible to change the HOM by modifying one or more of these parameters. For an inter-frequency handover, i.e. a handover from one frequency to another, a similar formula is used.
Another configurable parameter in relation to handover is the Time-to-trigger. Time-to-trigger is the time period required before triggering a handover attempt. If the user equipment experiences a better radio quality towards the target base station than towards the serving base station during this time period, then a handover attempt is triggered.
A problem with handover is handover oscillation. Handover oscillation is a behavior of a user equipment, where the user equipment during a short time period does handover from one cell to another and then back again one or several times.
The drawback of this behavior is:
                An increased risk for handover failure. Assuming that each handover has an inherent risk for failure, a larger number of handovers will increase the number of failures. There is a tradeoff here. If the handovers are delayed too much the risk for handover failure also increases.        A reduced throughput due to temporary loss of radio link. Also regarding throughput there is a tradeoff situation. If the handovers are delayed too much, the user equipment will in average operate in worse radio conditions, giving lower throughput.        An increased load on network elements, mainly eNodeB and Mobility Management Entity, MME, and transmission interfaces, S1 and X2. Each handover will trigger communication between the MME and the base station as well as between base stations, which will take up resources in the wireless communication network.On the other hand, if handover oscillation takes place due to varying radio quality or radio conditions, rather than defects in user equipment or RAN implementation, there also is a gain from handover oscillation. The user equipment opportunistically and repeatedly finds the best radio conditions, and thus increases the maximal throughput.        
One way to minimize handover oscillation is to configure the communication network with high HOMs and long handover time-to-trigger. This would delay and decrease the number of handovers generally, but as handovers are an essential function for user equipment mobility this would make the user equipments in average operate in worse radio conditions, and more often end up in situations where the radio link disconnects.
A handover oscillation may be defined as shown in FIG. 1. If T<Tosc, then the handover is considered as an oscillation handover. Or in other words, if the time period, T, from a first handover, from a cell A to a cell B, to a second handover, from a the cell B to the cell A, is shorter than a predefined time period Tosc then handover oscillation may be declared.
A measurement related to user equipment oscillation is oscillation rate. There is an upper boundary for an acceptable oscillation rate originating from e.g., core network load. Also the oscillation rate is related to end-user performance. On one hand oscillation are harmful as this induces additional signalling and delays, and on the other hand, oscillations allow the user to be connected to the best cell. This needs to be balanced in order for the end-user to experience the best performance.
An issue is how to set the parameter Tosc shown in FIG. 1, and thus how to determine if a handover is an oscillating handover or not. If Tosc is set too high handovers will not be determined as oscillating handovers even if they cause unnecessary control signalling load, or decrease end-user performance. If Tosc is set too low, also beneficial handovers are determined as oscillating handovers.
This will lead to that some handovers are determined to be handover oscillations in spite that they improve the throughput of the network, or that some handovers are not determined to be handover oscillations in spite that they decrease the throughput of the network, or both.